Equiatomic Binary Alloys Research Articles

Designing a Refractory Binary Alloy with Low Oxygen Solubility

Refractory metals such as Ti, Zr, Hf, V, Nb, and Ta can dissolve oxygen up to 33 %, 29 %, 20 %, 17 %, 9 %, and 5.7 %, respectively, at high temperatures. The high solubility of oxygen causes brittleness in these metals. When designing a refractory high entropy alloy, efforts should be focused on reducing oxygen solubility. We find that the high solubility of oxygen is related to the thermodynamic stability of oxygen interstitials in pure refractory metals. To understand the impact of alloy composition on oxygen solubility in refractory alloys, we examine the thermodynamic stability of oxygen interstitials across various refractory binary alloys. Using first-principles Density Functional Theory (DFT), we calculate the Oxygen Interstitial Formation Energy (OIFE) of nine refractory metals and their 24 equiatomic binary alloys. The OIFE is determined by the bond strength between oxygen and the nearest neighbor (NN) metal atoms. It is a strong function of the nature of NN atoms that surround the interstitial site rather than the overall composition of the alloy. For a given alloy, the OIFE can vary based on the NN atoms around the oxygen interstitial. These observations indicate that an alloy with even a small amount of metal of high oxygen affinity (or highly negative OIFE), like Ti, can result in the high stability of oxygen in specific interstitial sites where the nearest neighbors are predominantly Ti atoms. It also explains Re as a choice of alloying elements to ductilize W instead of Ti. Although both Ti and Re alloying in W similarly improve its ductility, Ti’s tendency to stabilize oxygen interstitials makes the W-Ti alloy brittle, while Re does not stabilize oxygen interstitials.

Ion irradiation induced surface composition modulation in equiatomic binary alloys

Composition modulation by Ar bombardment on the Co0.5Cu0.5 alloy and the CoAl B2 phase was investigated on atomic scale by molecular dynamics simulation. The Ar bombardment on alloys at 300 K revealed that this bombardment induces a surface composition modulation in the layer-by-layer mode. In both the Co0.5Cu0.5 alloy and the CoAl B2 phase, the element of higher-sputtering yield is accumulated on the top surface layer, whereas it is depleted in lower layers, which is puzzling in the framework the conventional sputtering theory. A phenomenological kinetic model derived from the MD simulation results considering both the rearrangement and the sputtering of the substrate atoms successfully demonstrated that the rearrangement of the substrate atoms plays a significant role in the observed composition modulation.

Short-Range-Order for fcc-based Binary Alloys Revisited from Microscopic Geometry

Short-range order (SRO) in disordered alloys is typically interpreted as competition between chemical effect of negative (or positive) energy gain by mixing constituent elements and geometric effects comes from difference in effective atomic radius. Although we have a number of theoretical approaches to quantitatively estimate SRO at given temperatures, it is still unclear to systematically understand trends in SRO for binary alloys in terms of geometric character, e.g., effective atomic radius for constituents. Since chemical effect plays significant role on SRO, it has been believed that purely geometric character cannot quantitatively explain the SRO trends. Despite these considerations, based on the density functional theory (DFT) calculations on fcc-based 28 equiatomic binary alloys, we find that while convensional Goldschmidt or DFT-based atomic radius for constituents have no significant correlation with SRO, atomic radius for specially selected structure, constructed purely from information about underlying lattice, can successfully capture the magnitude of SRO. These facts strongly indicate that purely geometric information of the system plays central role to determine characteristic disordered structure.

Novel mechanism for order patterning in alloys driven by irradiation

Kinetic Monte Carlo simulations have been performed to investigate the evolution of ordered domains in model alloys under irradiation. The alloys investigated were equiatomic binary alloys on a simple square lattice with first and second nearest-neighbor interactions, chosen so that a $2\ifmmode\times\else\texttimes\fi{}2$ ordered structure is the equilibrium phase below a critical order-disorder transition temperature ${T}_{\mathrm{c}}$. The ratio of second to first nearest-neighbor interactions $R$ was varied from 0 to 0.45 to explore the effect of the thermodynamic frustrations induced by the proximity of the $2\ifmmode\times\else\texttimes\fi{}1$ phase boundary, which occurs at $R=0.5$ for $T=0$. The atomic mixing produced by nuclear collisions was modeled by forcing the ballistic exchange of pairs of atoms at a controlled rate ${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{b}}$. This disordering process competed with thermodynamic reordering, resulting in nonequilibrium steady states. Two trivial steady states were found, a disordered state at high ${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{b}}$ and low $T$, and a long-range ordered state at low ${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{b}}$ and low $T$. In the $R=0.45$ alloy, however, a third steady state was identified at intermediate ${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{b}}$ and $T$ values, where multiple long-range ordered domains coexisted dynamically. It is shown that this state of patterning of order resulted from the coupling of the thermodynamic frustrations present in that alloy with the disorder introduced by irradiation. The practical relevance of this novel mechanism for patterning of order under irradiation is discussed in the context of recent observations of domain coexistence in irradiated ${\mathrm{Cu}}_{3}\mathrm{Au}$.

A high accuracy diffusion kinetics formalism for random multicomponent alloys: application to high entropy alloys

In this paper, a new, lighter, version of the highly accurate Moleko, Allnatt and Allnatt formalism for describing both tracer (self) and collective diffusion kinetics in multicomponent random alloys is presented. Verification of the resulting expressions is performed by means of kinetic Monte Carlo simulation. The accuracy of the new formalism is much higher than that of the combined Manning and Holdsworth and Elliott formalism discussed recently. Using this formalism the possible range of the tracer diffusion ratio of the highest to the lowest atomic component is examined for equiatomic (or near equiatomic) binary, ternary, quaternary and quinary alloys. It is shown that in the random alloy model, the correlation effect is the highest with a reduction of the fastest tracer diffusion by 40–55%, when moving from two pure metals to their equiatomic binary alloy. By adding the third component (with an intermediate mobility) this effect can be further increased with a possible total reduction of the fastest tracer diffusion by up to 70% (depending on the combinations of mobilities), while adding the fourth component brings this reduction up to 80% and with a possible maximum of up to 85% reduction for the 5-component alloy (again depending on the combinations of mobilities). But the slowest diffusing components are not affected by this. This suggests that kinetics arguments alone are not enough for explaining the sluggish diffusion observed of all atomic components in (equiatomic) high-entropy alloys.

Ion irradiation induced defect evolution in Ni and Ni-based FCC equiatomic binary alloys

In order to explore the chemical effects on radiation response of alloys with multi-principal elements, defect evolution under Au ion irradiation was investigated in the elemental Ni, equiatomic NiCo and NiFe alloys. Single crystals were successfully grown in an optical floating zone furnace and their (100) surfaces were irradiated with 3 MeV Au ions at fluences ranging from 1 × 1013 to 5 × 1015 ions cm−2 at room temperature. The irradiation-induced defect evolution was analyzed by using ion channeling technique. Experiment shows that NiFe is more irradiation-resistant than NiCo and pure Ni at low fluences. With continuously increasing the ion fluences, damage level is eventually saturated for all materials but at different dose levels. The saturation level in pure Ni appears at relatively lower irradiation fluence than the alloys, suggesting that damage accumulation slows down in the alloys. Under high-fluence irradiations, pure Ni has wider damage ranges than the alloys, indicating that defects in pure Ni have high mobility.

SURFACE MODIFICATION OF NITINOL BY CHEMICAL AND ELECTROCHEMICAL ETCHING

In this paper, Nitinol, an equiatomic binary alloy of nickel and titanium, was surface modified for its potential biomedical applications by chemical and electrochemical etching. The main objective of the surface modification is to reduce the nickel content on the surface of Nitinol and simultaneously to a rough surface microstructure. As a result, better biocompatibility and better cell attachment would be achieved. The effect of the etching parameters was investigated, using scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectrometry (EDX) and X-ray photoelectron spectrometry (XPS). The corrosion property of modified Nitinol surfaces was investigated by electrochemical work station. After etching, the Ni content in the surface layer has been reduced and the oxidation of Ti has been enhanced.

Electron Dispersion in Liquid Alkali and Their Alloys

Ashcroft's local empty core (EMC) model pseudopotential in the second-order perturbation theory is used to study the electron dispersion relation, the Fermi energy, and deviation in the Fermi energy from free electron value for the liquid alkali metals and their equiatomic binary alloys for the first time. In the present computation, the use of pseudo-alloy-atom model (PAA) is proposed and found successful. The influence of the six different forms of the local field correction functions proposed by Hartree (H), Vashishta–Singwi (VS), Taylor (T), Ichimaru–Utsumi (IU), Farid et al. (F), and Sarkar et al. (S) on the aforesaid electronic properties is examined explicitly, which reflects the varying effects of screening. The depth of the negative hump in the electron dispersion of liquid alkalis decreases in the order Li → K, except for Rb and Cs, it increases. The results of alloys are in predictive nature.

The effect of a dielectric coating on the surface energy of thin metal films of the Al-Li alloy system

The surface energy σ of a thin film of an equiatomic binary alloy Al0.5Li0.5 contacting with a dielectric medium is calculated using the method of electron density functional. It is established that the difference Δσ = σ0 − σ(e) (σ0 is the initial surface energy and e is the permittivity) is an almost linear function of e−1.

On the relationship of high coercivity and L1 ordered phase in CoPt and FePt thin films

The microstructure and the room-temperature hysteretic magnetic properties of sputtered, 10 nm thin films of equiatomic binary alloys of CoPt and FePt were characterized using transmission electron microscopy (TEM) and a superconducting quantum interference device (SQUID) magnetometer. A transformation from an atomically disordered, face-centered-cubic structure to the L10 ordered structure occurred during postdeposition annealing and was characterized using digital analysis of dark-field TEM images. The transformation was observed to follow first-order nucleation and growth kinetics, and the ordered volume fraction transformed was quantified at numerous points during the transformation. The ordered volume fraction was then compared to the magnetic coercivity data obtained from the SQUID magnetometer. In contrast to the relationship most commonly described in the literature, that the highest coercivity corresponds to a two phase ordered/disordered mixture, the maximum value for coercivity in this study was found to correspond to the fully ordered state. Furthermore, in samples that were less than fully ordered, a direct relationship between ordered volume fraction and coercivity was observed for both CoPt and FePt. The proposed mechanism for the high coercivity in these films is an increasing density of magnetic domain wall pinning sites concurrent with an increasing fraction of ordered phase.

Effect of temperature on the structure and electronic properties of the liquid alloy K–Sb

We have carried out elastic neutron scattering measurements on the liquid equiatomic binary alloy K–Sb. The derived structure factor, S( Q), has a prepeak at about 1.1 Å −1 indicative of large structural units. With increasing temperature to about 1800 K we find remarkably small changes in S( Q). We used the reverse Monte Carlo simulation method to explore details of the liquid structure in real space and found evidence to ascribe the prepeak in S( Q) to Sb–Sb correlations of Sb chain fragments. The electrical conductivity has a distinct increase from 0.8 to 40 Ω −1 cm −1 at T ≈ 825 K, and a conductivity of approximately 350 Ω −1 cm −1 is reached at 1673 K.

Grain growth in ultrathin films of CoPt and FePt

The microstructure of sputtered 10-nm thin films of equiatomic binary alloys of CoPt and FePt was characterized using transmission electron microscopy (TEM). Grain growth kinetics was examined using manual and digital analysis of bright-field TEM images and was seen to take two stages during annealing in these films. A rapid growth stage concurrent with the formation of a [111] fiber texture was observed to occur within the first 5–10 min of annealing, followed by a much slower growth stage after the fiber texturing was well advanced. Differences in grain growth rate and ultimate grain size were also observed to depend on heating rate.

A Study On High Coercivity And Lio Ordered Phase In Copt And Fept Thin Films

ABSTRACTThis study relates the microstructure of equiatomic binary alloys of CoPt and FePt with their room-temperature hysteretic magnetic properties, particularly their high coercivity. A transformation from an atomically disordered, face-centered-cubic structure to the Li0ordered structure occurred during post-deposition annealing and was characterized using digital analysis of dark-field transmission electron microscopy (TEM) images. The transformation was observed to follow first-order nucleation and growth kinetics, and the ordered volume fraction transformed was quantified at numerous points during the transformation. The ordered volume fraction was then compared to the magnetic coercivity data obtained from a superconducting quantum interference device (SQUID) magnetometer. Although the relationship most commonly described in the literature is that the highest coercivity corresponds to a two phase ordered/disordered mixture, the maximum value for coercivity in this study was found to correspond to the fully ordered state. Furthermore, in samples that were less than fully ordered, a direct relationship between ordered volume fraction and coercivity was observed. The proposed mechanism for the high coercivity in these films is an increasing density of magnetic domain wall pinning sites concurrent with an increasing fraction of ordered phase.

A Study on High Coercivity and L10Ordered Phase in CoPt and FePt Thin Films

ABSTRACTThis study relates the microstructure of equiatomic binary alloys of CoPt and FePt with their room-temperature hysteretic magnetic properties, particularly their high coercivity. A transformation from an atomically disordered, face-centered-cubic structure to the Li0ordered structure occurred during post-deposition annealing and was characterized using digital analysis of dark-field transmission electron microscopy (TEM) images. The transformation was observed to follow first-order nucleation and growth kinetics, and the ordered volume fraction transformed was quantified at numerous points during the transformation. The ordered volume fraction was then compared to the magnetic coercivity data obtained from a superconducting quantum interference device (SQUID) magnetometer. Although the relationship most commonly described in the literature is that the highest coercivity corresponds to a two phase ordered/disordered mixture, the maximum value for coercivity in this study was found to correspond to the fully ordered state. Furthermore, in samples that were less than fully ordered, a direct relationship between ordered volume fraction and coercivity was observed. The proposed mechanism for the high coercivity in these films is an increasing density of magnetic domain wall pinning sites concurrent with an increasing fraction of ordered phase.

Comment on the paper 'The effect of short-range order on the vibrational entropy of one-dimensional chains'

For original paper see J.A.D. Matthew et.al., ibid., vol.13, p.581 (1983). Matthew et. al. derived an expression for the entropy change due to disordering of an ordered one-dimensional equiatomic binary alloy at high temperatures, using the harmonic approximation and assuming a fixed-end boundary. As a first step towards the calculation of entropy changes due to disordering of ordered two- and three- dimensional binary alloys (Bakker 1982, Waegemaekers and Bakker 1983) the present author had already studied the linear chain and was able to derive in a very simple and straightforward way the exact expression for the entropy change of a binary linear chain which changes its state I of order to a state II of order.

Crystallographic and magnetic properties of pseudobinary alloys based on cobalt-platinum

Crystallographic and magnetic properties of a range of pseudobinary (FeNi)Pt alloys based on the equiatomic binary alloy CoPt have been investigated and the results have been combined with those of previously reported investigations on (CoFe)Pt and (CoNi)Pt alloys. X-ray crystallographic investigations show that the alloys exhibit the disorder-order transformation found for CoPt. Measurements of Curie temperatures and magnetic hysteresis parameters show that substitutions for cobalt in the simple CoPt alloy adversely affect the magnetic hardness of the alloy.

Band model for order-disorder transition in equiatomic binary alloys

In equiatomic binary alloys, by using simple approximations on a model hamiltonian (synthesis of those of Hubbard and Soven with interatomic interactions) we conclude that the order-disorder transition is generally sharper than with the Ising model, and can even be of first order.